Nickel Defense

What is Nickel Defense?

A defensive alignment that uses five defensive backs. The nickel
is one of the most common alternative defensive alignments in use
today, and it is considered to be primarily a passing defense.
The defense uses an extra defensive back to combat offenses that
have a dynamic tight end or use three wide receiver sets regularly.
The nickel defense is more effective against the pass due to the
extra defensive back, but it can be susceptible to running plays
because the defense will have one less linebacker or defensive
lineman than usual.

Sporting Charts explains Nickel Defense

The nickel defense is a very common defense in today's game,
particularly due to the evolution of passing offenses in recent
years. Many teams will utilize extra wide receivers, and defenses
have had to turn to extra defensive backs in order to minimize the
coverage concerns that can emerge when linebackers have to guard
wide receivers. Nickel defenses can also be handy if the team has a
particularly effective tight end who causes matchup problems for
the linebackers.